found that proxy... https://umbraco.com/blog/integrating-umbraco-cms-with-hubspot-forms/

The proxy is hosted here: https://hubspot-forms-auth.umbraco.com/, but you are more than welcome to try it also yourself on your local development environment using the GitHub source code. Umbraco is managing and hosting the solution, but we are open to any questions or related feedback you might have.